In a cognitive radio system, which is a radio communication system in which radio parameters are adaptively changed according to a surrounding radio environment, a surrounding radio environment is cognized and radio parameters are optimized according to the cognized radio environment.
The cognitive radio system is disclosed in, for example, Patent Document 1 (JP 2007-88856 A). In the cognitive radio system disclosed in Patent Document 1, a plurality of radio transceivers belonging to a given group perform radio communication by using a given communication scheme, and a plurality of radio transceivers belonging to another group perform radio communication by using another communication scheme. In a case where one of the plurality of radio transceivers belonging to the given group is a television broadcast station, and the other radio transceivers are television receivers, one of the plurality of radio transceivers belonging to the other group is a cognitive radio base station, and the other radio transceivers are cognitive radio terminals. Then, the cognitive radio base station and the cognitive radio terminals perform communication by using times and frequencies which are not used for television broadcasting. Accordingly, there is no need to consider interference between the given group and the another group.
In view of this, as a secondary system, a cognitive radio system shares a frequency band that is the same as a frequency band allocated to an existing radio communication system (hereinafter, may be referred to as primary system), to thereby improve the frequency usage efficiency.
In general, from a viewpoint of protecting the existing system, a basis of sharing a frequency band is that the primary system preferentially uses a frequency band that is allocated thereto in advance and the secondary system does not influence the primary system. Therefore, the secondary system is required to avoid interference to the primary system by, for example, using a frequency band that is not used by the primary system, or controlling transmission power to be smaller than an interference amount allowed by the primary system. In order to achieve this, the secondary system needs to correctly recognize a status of frequency band usage of the primary system, which is to be used by the secondary system.
Situations in which the secondary system recognizes the status of frequency band usage of the primary system may be classified into two main types. One type corresponds to a case in which, before performing communication, the secondary system detects communication of the primary system in a wide candidate frequency band that may be shared with the secondary system. The other type corresponds to a case in which the secondary system detects the primary system that has started communication in a frequency band that is being used by the secondary system. In both of the cases, if the secondary system detects communication of the primary system, the secondary system needs to take a measure to avoid the interference to the primary system in the corresponding frequency band.
As a specific method of detecting existence of communication of the primary system, there is spectrum sensing in which a radio apparatus of the secondary system detects a signal in its surrounding. Spectrum sensing includes a power detection method which is used for determination performed based on a magnitude of a received signal power determined based on a time average, and a method in which a feature amount contained in a transmitted signal of the primary system is used for detection. As the feature amount of the signal, cyclostationarity, a pilot signal, or the like contained in the transmitted signal of the primary system may be used (see, for example, Non-patent Document 1).
Non-patent Document 1: D Cabric, S M Mishra, and R W Brodersen, “Implementation issues in spectrum sensing for cognitive radios,” Proc of the Thirty-Eighth Asilomar Conference on Signals, Systems and Computers, November 2004.
However, spectrum sensing performed by individual secondary system radio apparatus involves a problem that secure detection of the primary system is difficult due to influences of a surrounding radio propagation environment, including fading, shadowing, and distance attenuation. As a solution thereto, there has been studied a cooperative sensing scheme in which a detection accuracy is increased by combining spectrum sensing functions of a plurality of radio apparatus (see, for example, Non-patent Document 2).
Non-patent Document 2: Shridhar M Mishra, Anant Sahai and Robert W Brodersen, “Cooperative Sensing among Cognitive Radios,” Proc of IEEE International Conference on Communications (ICC) 2006.)
FIG. 1 is a system conceptual diagram of cooperative sensing. In the example illustrated in FIG. 1, as a primary system, a radio apparatus 100 and a radio apparatus 110 communicate with each other. As a secondary system, radio apparatus 200, 210, 220, and 230 share the same frequency band as that of the primary system. Here, the radio apparatus 200, 210, 220, and 230 of the secondary system constitute a cooperative group for collaboration thereamong in terms of a spectrum sensing function. Among the radio apparatus, the radio apparatus 200 functions as a master node, controls each of the radio apparatus in the cooperative group, and makes, as the cooperative group, a determination on detection of whether or not communication of the primary system exists. The other radio apparatus 210, 220, and 230 function as slave nodes that perform a cooperative sensing operation according to an instruction issued by the master node. Therefore, hereinafter, the radio apparatus 200 may be referred to as master node 200, and the radio apparatus 210, 220, and 230 may be referred to as slave modes 210, 220, and 230, respectively.
A basic operation of cooperative sensing is as follows. The plurality of slave nodes 210, 220, and 230 belonging to the cooperative group perform spectrum sensing on a target frequency band. The plurality of slave nodes 210, 220, and 230 notify the master node 200 of results thereof as detection information. The master node 200 integrates notified detection information pieces to determine whether or not the communication of the primary system exists in the target frequency band.
In this manner, in cooperative sensing, influences of a radio propagation environment may be alleviated by virtue of a configuration in which the plurality of secondary system radio apparatus are spatially distributed. Therefore, detection performance may be improved in comparison with spectrum sensing performed by individual secondary system radio apparatus.